The effects of delay distortion, specifically those associated with fiber chromatic dispersion, have limited bit rates and repeater spans of conventional lightwave communication systems. The underlying mechanism that causes this limitation is intersymbol interference. In addition, direct modulation of single-longitudinal mode semiconductor lasers causes a transient wavelength chirping (laser chirp), which in combination with the chromatic dispersion of the fiber, causes increased intersymbol interference. Laser chirp alone has no deleterious effect on direct detection optical links unless there is dispersion in the transmission medium, while fiber dispersion alone, with no laser chirp present, can still limit the bit rate and repeater span resulting from associated intersymbol interference. The effects of laser chirp combined with fiber dispersion are discussed in articles by P. J. Corvini et al., J. Lightwave Tech., Vol. LT-5, No. 11 pp. 1591-5 (1987), D. A. Atlas et al., Optics Lett., Vol. 13, No. 11, pp. 1035-7 (1988), and J. C. Cartledge et al., J. Lightwave Tech., Vol. 7, No. 3, pp. 568-73 (1989). Further, the effects of fiber dispersion alone are discussed, for example, in the article by A. F. Elrefaie et al., J. Lightwave Tech., Vol. 6, No. 5, pp 704-6 (1988).
With the trend toward higher bit rates and longer repeater spans, various equalization techniques have been proposed to reduce the effects of laser chirp. Approaches include use of optical pulse shaping and external modulation techniques. See, D. Le Clerc et al., Proceedings of 1985 IOOC-ECOC, pp. 775-8. Additionally, techniques employing semiconductor laser amplifiers have been used to impose a chirp on a transmitted optical signal to counteract the combined effects of fiber dispersion and laser chirp. See, N. A. Olsson et al., Electron. Lett., Vol. 25, No. 9 pp. 603-5 (1989).
The above approaches attempt to reduce the effects of laser chirp directly at the transmitter source, i.e., the laser. An alternative approach focuses on reducing the resulting effects of dispersion directly at the receiver by utilizing a transmission medium having a strong chromatic dispersion opposite to that of the transmission fiber. Particularly, singlemode fibers, dispersive gratings and optical tapped delay lines have been demonstrated to reduce the effects of laser chirp and fiber dispersion. See, D. Marcuse, Appl. Optics, Vol. 20, No. 4 pp. 696-9 (1981) and L. B. Jeunhomme, Single Mode Fiber Optics, pp. 213-5 (1983).
In all of the approaches above relating to equalization, there exist problems in achieving low optical power penalty while maintaining low cost and simplicity.